The chemical theory rests on three fundamental physiological principles: (1) the response of cells to stimuli, in this connection _specific chemical stimuli_, (2) the presence within cells of _specific chemical groups_ which combine with chemical stimuli and thus enable them to act on the cell, which groups Ehrlich has named _receptors_, and (3) the “_over-production_” activity of cells as announced by Weigert. 1. That cells respond to stimuli is fundamental in physiology. These stimuli may be of many kinds as mechanical, electrical, light, thermal, chemical, etc. The body possesses groups of cells specially developed to _receive_ some of these stimuli--touch cells for mechanical stimuli, retinal cells for light, temperature nerve endings for thermal, olfactory and gustatory cells for certain chemical stimuli. _Response_ to chemical stimuli is well illustrated along the digestive tract. That the chemical stimuli in digestion may be more or less specific is shown by the observed differences in the enzymes of the pancreatic juice dependent on the relative amounts of carbohydrates, fats, or proteins in the food, the specific enzyme in each case being increased in the juice with the increase of its corresponding foodstuff. The cells of the body, or certain of them at least, seem to respond in a specific way when substances are brought into direct contact with them, that is, without having been subjected to digestion in the alimentary tract, but injected directly into the blood or lymph stream. Cells may be affected by stimuli in one of three ways: if the stimulus is too weak, there is no effect (in reality there is no “stimulus” acting); if the stimulus is too strong, the cell is injured, or may be destroyed; if the stimulus is of proper amount then it excites the cell to increased activity, and in the case of _specific chemical stimuli_ the increased activity, as mentioned for the pancreas, shows itself in an _increased production of whatever is called forth by the chemical stimulus_. In the case of many organic chemicals, the substances produced by the cells under their direct stimulation are markedly specific for the particular substance introduced.